To my knowledge... calibre doesn't yet have a way of dealing with the DRM-free versions of the stand-alone KF8 ebook files that are starting to find their way into many people's libraries (and will only be steadily increasing). The viewer won't handle them properly and they can't be converted.
Nor can calibre convert anything to
the KF8 format, yet (that I know of).
I imagine that will eventually change, though.